Method for the manufacture of dislocation-free, single-crystal gallium arsenide rod

ABSTRACT

An improved method for drawing dislocation-free, single crystal gallium arsenide rods in which the drawing vessel is filled with an inert gas of a preselected conductivity at a predetermined pressure, additional arsenic is added to maintain a desired partial pressure, and the melt temperature is controlled to initially draw a &#39;&#39;&#39;&#39;narrow-neck&#39;&#39;&#39;&#39; and then decreased until a crystal of the desired diameter is being drawn. The initial drawing of the narrow neck causes the dislocations to travel in the direction of their growth and disappear, after which a crystal of the desired diameter with dislocations may be drawn.

United States Patent 1191 1111 3,819,421

Merkel et al. June 25, 1974 4 METHOD FOR THE MANUFACTURE OF 3,344,071 9/1967 Cronin 252/623 GA DISLOCATION FREE, SINGLE CRYSTAL 3,488,57 141970 Koffer 234301 SP X 3,507, 25 i970 Deyris 23 BM SP X GALLIUM ARSENIDE ROD 3,627,499 12/1971 Le Duc et al. 23/301 SP [75] Inventors: Hans Merkel; Hans-Jochen Wolf,

both of Erlangen, Germany Primary Examiner-G. T. Ozaki [73] Assrgnee: Siemens Aktiengesellschaft, Attorney, Agent, or Firm-Hugh A. Chapin Munich,- Germany [22] Filed: Feb. 22, 1973 57 ABSTRACT [21] Appl. No.: 334,935 1 An improved method for drawing dislocation-free, sin- A gle crystal gallium arsenide rods in which the drawing [30] Forelgn pphcatmn Pnomy Data vessel is filled with an inert gas of a preselected con- Mar. 1, 1972 Germany 2209869 ductivity at a predetermined pressure, additional arse U S Cl 148/1 6 148/171 1487172 nic is added to maintain a desired partial pressure, and

the melt temperature is controlled to initially draw a 23/301 252/6.2'3 GA narrow-neck and then decreased until a crystal of [51] Int. Cl B01 l7/ld the desired diameter is being drawn The initial draw [58] Field of Search 14871.6, l7ll73, ing of the narrow neck causes the dislocations to 23/301 252/623 GA travel in the direction of their growth and disappear,

after which a crystal of the desired diameter with dis- [56] References Cited locations may be drawn.

UNITED STATES PATENTS 2,962,363 ll/l960 Martin 23/301 SP 7 Claims, 2 Drawing Figures as i ::l -15 1a a Q n PATENTED JUN 2 5 I974 SHEET 1 OF 2 PATENTEU I974 3.819.421

SHEET 2 OF 2 l N L.) b l|||||||1|l11|xlljlllllllllllllllull METHOD FOR THE MANUFACTURE OF DISLOCATION-FREE, SINGLE-CRYSTAL GALLIUM ARSENIDE ROD BACKGROUND OF THE INVENTION This invention relates to the manufacture of crystals in general and more particularly to an improved method for manufacturing single crystal gallium arsenide rods.

Recent developments in the electronics art have led to the gallium arsenide crystal becoming increasingly more important. These developments include the discovery of the injection laser, world-wide efforts to exploit the Gunn effect, the increased manufacture of light-emitting diodes and other opto-electronic coupling elements, the application of high-resistivity gallium arsenide as an electrical insulating substrate for epitaxial devices, and other similar applications. In each of these applications, the perfection of the crystals of the semiconductor material, gallium arsenide, is of extremely great importance.

In the prior art, high-quality crystals of a semiconductor material, particularly those in which, at the melting temperature of the compound, the partial pressures of their components in the compounds were different, as is the case with gallium arsenide, were produced in a closed vessel by drawing from a melt using the Czochralski method. An example of such an apparatus is that shown in the German Pat. No. 1,044,769. The single crystal rods of gallium arsenide which are produced by the method described in the German patent above, are frequently not satisfactory. In that method, which includes using a vessel sealed at the top and at the bottom, magnetic coupling to draw and rotate the crystal, and a particular type of seed mount, crystals which have an excessive number of dislocations are formed. These imperfections result because dislocations present in the seed crystal continue to grow in the crystal and are further augmented by dislocations which are newly formed at the interface between the seed and the crystal. New dislocations are also incorporated into the crystal by excessive separa tion rates, i.e., a large increase of the diameter over a short axial distance.

An improved method of drawing dislocation-free gallium arsenide crystals is disclosed by A. Steinemann and U. Zimmerli in the Journal of Physics and Chemistry of Solids, Supplement No. 1, page 81, (1967). The method described in there is a narrow-neck drawing method such as that disclosed by G. Ziegler in a paper On the Formation of Dislocation-Free Silicon Single Crystals, Zeitschrift fuer Naturforschung, vol. 16a, No. 2, page 2l9 (I961). The Steinemann and Zimmerli method comprises reducing the diameter of the growing crystal immediately after drawing to l to 2 mm. The small diameter is maintained over a crystal length of to mm. and then the narrow neck is gradually enlarged to the desired crystal diameter. In the narrowneck zone the dislocations grow toward the crystal surface and disappear. The lower part of the narrow neck is therefore completely free of dislocations and the remaining problem of crystal drawing then consists only of preventing the formation of new dislocations.

Since the majority of the dislocations formed in the drawing process are generated by plastic deformation due to thermal stresses, the thermal conditions in the vicinity of the solidification front, i.e., where the substance changes from a liquid to a solid, are of great importance. According to the statements on page 82 of the Steinemann and Zimmerli article, the following conditions must be fulfilled to obtain crystal growth which is free of dislocations: l) a small axial temperature gradient must be maintained at the transition from solid to liquid, (2) the radial heat flow in the crystal must be as small as possible, and (3) the solid to liquid transition zone must be nearly parallel. To obtain these conditions Steinemann and Zimmerli replaced the conventional open melting crucible (as is described in German Pat. No. 1,044,769) with an almost completely closed crucible which included a radiation shield. The lid on the closed crucible has an opening in the center of a diameter sufficiently large for the crystal to be drawn therethrough into the upper portion of the drawing vessel. In their article on page 82 in the lefthand column, bottom and the right-hand column, top, Steinemann and Zimmerli state that the cover with the radiation shield is an absolute necessity for drawing dislocation-free gallium arsenide single crystals.

The apparatus which they described has a number of disadvantages. For example the cover makes observation of the crystal difficult, and is the source of defects such as contamination.

7 Thus, there is a need for a method of forming dislocation-free single crystals of gallium arsenide which avoids the disadvantages of the prior art methods.

SUMMARY OF THE INVENTION The present invention provides such a method in which single crystal gallium arsenide rods which are free from dislocations can be produced with a simple and inexpensive apparatus. In the disclosed method the gallium arsenide crystals are drawn within an enclosed quartz tube, from a gallium arsenide melt in a graphite crucible which is inductively heated. The mounting of the seed crystal, the changing of the height and the rotation of the seed crystal are achieved by means of magnetic coupling in the manner described in German Pat. No. 1,044,769. The temperature within the quartz tube is controlled so that over a distance of about 3 cm the crystal is reduced to a diameter as small as possible and then is gradually increased to a desired diameter. Prior to sealing the drawing vessel, i.e., the quartz tube, the area over the gallium arsenide crucible is charged with an inert gas or an inert gas mixture at a pressure of0.2 to 0.8 kg/cm (0.2 to 0.8 X l0 N/m The charging gas is selected to have a thermal conductivity of 0.18 mW cmK' at 300 K and 0.3 mW cml( at l,500 K. In addition, the arsenic addition is proportioned so that the partial pressure of the arsenic during the drawing process is 0.8 to 1.5 kg/cm (0.8 to 1.5 X 10 N/m In drawing the crystal a seed crystal diameter of between 2 and 10 mm. is used.

By taking the steps outlined above, the thermal stresses which would otherwise be present are prevented due to a favorable heat flow. Because of the small temperature gradient the crystal will grow well without plastic deformation. Because of this a singlecrystal gallium arsenide rod with at most a few dislocations will be formed.

The gases and/or gas mixtures with the low heat conductivity mentioned above will preferably be krypton and zenon although a mixture of these gases or mixtures of one of these gases with argon and in some circumstances with nitrogen can also be used. The thermal shielding provided by these gases or gas mixtures provides an axial temperature gradient at the transition between solid and liquid and a radial heat flow in the crystal and in the melt which are very small. As a consequence, the transition zone between the solid and liquid phase is perfectly flat. The apparatus disclosed for practicing this method is much simpler than that described by Steinemann and Zimmerli. In addition, the introduction of impurities which result from the lid and radiation shield of graphite at the growing crystal is prevented.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross section view, partially in schematic form, of a typical apparatus for performing the crystal growing method of the present invention.

FIG. 2 is a diagram illustrating the temperature dependence of the thermal conductivity of several gases.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Prior to describing the method of the present invention, some characteristics of the gallium arsenide used in the present invention should be noted. At its melting point of 1,238 C., gallium arsenide has an arsenic vapor pressure of l kglcm In practicing the present invention it is desirable to work with an arsenic vapor pressure in the range of 0.8 to 1.5 kg/cm preferably 1.3 kg/cm To obtain this vapor pressure, the drawing must be done within a vessel which is enclosed, and the addition of some arsenic is required. The vessel will preferably be of fused quartz. During the drawing operation the lowest temperature of the vessel wall must be maintained at at least 680 C. It is known that the vapor pressure of arsenic in a closed container above a base of arsenic is l kg/cm at 615 C. and 1.3 kg/cm at 628 C. (see Gmelins Handbuch der anorganischen Chemie, 8th Edition, 1952, System No. 17, Arsenic, page 134). From this it follows that when there is a body of arsenic present in the system, solid arsenic will precipitate at the wall of the vessel where its temperature drops below 628 C. Thus, in addition to the gallium arsenide which will be placed in the drawing crucible, an additional amount of arsenic can be deposited in the vessel such that it will be completely converted into vapor form at 628 C. in order that the arsenic vapor in the vessel has a pressure of 1.3 kg/cm These conditions will be maintained if part of the wall of the vessel is kept at 680 C. and this is the lowest temperature in the drawing system.

In performing the steps of the present invention, the crucible, from which the GaAs crystal is to be pulled, is filled or charged with Stoichiometrically structured, mostly in polycrystalline form, gallium arsenide which has been synthesized from the gallium and arsenic elements using well known methods. It should be noted, that it is also possible to perform the synthesis of the gallium arsenide in the drawing apparatus itself. To perform this synthesis, the required amount of gallium, accurately weighed, and which has been freed of oxide layers by a prior heat treatment, is placed in the crucible. A sufficient amount of Oxygen-free arsenic to form a stoichiometrically composed GaAs melt in the crucible is then deposited in the lower part of the drawing vessel along with enough excess arsenic to maintain an arsenic vapor pressure of 1.3 kg/cm with the minimum temperature of the vessel wall at 680 C.

The apparatus for pulling of a dislocation-free singlecrystal gallium arsenide rod in accordance with the method of the present invention is shown in FIG. 1. The vessel 1 in which the pulling is accomplished comprises a transparent fused quartz member having a length of 60 cm and an inside diameter of 4.5 cm. After each drawing operation, the vessel is cut at the point indicated as 2 on the figure to permit access to the pulling mechanism in the rod. The vessel will be useable a number of times [at least 20 times] having been made longer than is necessary for a single pulling operation. A guide tube 32 is attached to the top of the vessel 1 and is perfectly aligned along the axis of the vessel. The pulling mechanism comprises a drawing tube made of quartz glass which slips over the guide tube 32. Both guide tube 32 and the inside of tube 31 are carefully ground to provide a closely aligned fit between the two surfaces. Four segments of magnetic steel 33 are secured to the tube 31 at its outside at 3. These interact with the magnets 15 mounted outside the tube 1. in the manner described in the above referenced German patent, to pull and rotate the drawing tube 31. Secured to the base of the tube 31 is the mount 4 for the seed crystal. In the lower part of the vessel 1, is a graphite crucible 8 in which the gallium arsenide melt is deposited. If desired, in order to eliminate the influence of the graphite and its impurities, the entire crucible can be enclosed in fused quartz, or inserts of fused quartz, boron nitrite, aluminum nitrite, aluminum oxide or other suitable pure materials may be placed inside of the crucible and should be fit thereto as closely as possible.

The seed crystal mount 4, is composed of graphite or boron nitrite and is secured to the lower end of tube 31 with a quartz splint. The seed crystal 6 will preferably be 6 cm long and will have a (111) orientation; its lower half will be cylindrical and have a diameter of 4 mm and its upper half will be ground as a prism with a cross section of 4 mm by 4 mm. The prismatic portion of the seed crystal will be inserted very accurately into a corresponding shaped opening in the seed crystal mount 4. Eight screws made of graphite or boron nitrite will hold the seed crystal 6 in place in the crystal mount 4. The crucible 8 is formed by drilling a cylindrical graphite block, preferably one with a diameter of 4 cm and a height of 5 cm, to form a hole which is 4 cm deep and of a diameter such as to leave a wall thickness of 4 mm. With these dimensions the crucible will hold a little more than 60 grams of gallium arsenide melt. The remaining portion of the graphite block interacts with a high frequency magnetic field provided by coils outside the vessel to inductively heat the melt in the manner described in the above referenced German patent. The coils 13 which provide the high frequency field to the crucible 8 are water cooled in a manner described in the reference. Secured to the bottom of the vessel 1 is a graphite quartz socket 9 used to support the crucible 8.

Prior to beginning the drawing operation the vessel 1 will be cut in two sections at the line 2. The crucible 8 will be filled with 60 grams of gallium arsenide in the form of polycrystalline rods of suitable length. The crucible 8 is then placed on the quartz socket 9 in the lower part of the vessel 1. In the space 10 between the quartz socket 9 and the lower part of the quartz tube is placed the required excess arsenic to maintain the desired vapor pressure. For the present example 2.7 grams of arsenic would be used. The two halves of vessel 1 are then fused together, in exact vertical alignment with the end of the finally etched seed crystal placed carefully on the gallium arsenide rods located in the crucible 8. The vessel 1 is then evacuated by an evacuation stub 7 located at the top of the vessel. The portion of the vessel 1 above the crucible 8 is then heated for 1 hour at 620 C. The vessel is cooled under a vacuum and a mixture of krypton and xenon gas admitted until a pressure of 0.5 kg/cm is reached in the vessel. The vessel 1 is then sealed by closing off the evacuation tube 7 and is now ready for the drawing operation. The vessel 1 is then placed in a furnace installation which comprises a resistance furnace 11 extending over most of the upper secton of the vessel 1, a short resistance furnace 12, the induction heating coils 13, and a lower resistance furnace 14, which heats the base of the vessel 1. The resistance furnace 11 may, for example comprise a slotted Megapyr tube. This resistance furnace l1 heats the vessel down to the area near the crucible. At that point, heating is taken over by the short furnace 12 which has its heater windings arranged in a slanted or stair-like ceramic body to facilitate viewing the crystal as it is being drawn. The gallium arsenide is heated to its melting point by the inductive heating of the graphite crucible 8, with coils 13, which are coupled to a high frequency generator in a manner known in the art.

The entire vessel is brought to a temperature of approximately 650 C. by means of the resistance furnaces ll, 12 and 14 causing all the excess arsenic which was deposited in the portion 10 of the vessel to evaporate. The temperature of the furnaces 11 and 12 is then increased to 700 C. and that of furnace 14 to 680 C. The gallium arsenide in the crucible 8 is then melted by the high frequency inductive heating described above. The temperature of the gallium arsenide melt is controlled by means of a radiation pyrometer in a manner well known in the art.

When temperatures have stabilized, and the gallium arsenide is melted, the seed crystal is dipped into the melt and after temperature equilibrium is established, the pulling process is started. The magnets 15 are used to rotate the pulling tube 31 at a rotation speed of about rpm. The temperature of the melt is maintained at a level such as to cause the growing crystal to have a diameter which is decreased down to about 1 mm. This comprises the narrow-neck, and is then drawn out for approximately 15 to 20 mm.

After pulling the crystal for this distance, the neck of the crystal is free of dislocations. The dislocations will have traveled, following the direction of their growth, toward the lateral boundary of the narrow-neck and will have disappeared. The temperature of the gallium arsenide melt is then reduced slowly so that the crystal gradually becomes thicker again. When it has reached the desired diameter, for example, 15 mm, the drawing is continued with a constant pulling velocity of about 2 cm per hour at a rotation speed of 20 rpm. In this manner, a dislocation-free cylindrical crystal of 15 mm diameter with a length of 9-10 cm is formed.

The graph of FIG. 2 illustrates the conductivity of various gases over a range of temperatures and is helpful in selecting the gas or gases to be used in the drawing vessel.

Thus, a method of obtaining gallium arsenide crystals which are essentially free of dislocations, have improved purity and quality and are suited, due to their high crystal perfection, for the manufacture of semiconductor devices with optimum properties, for example, light emitting diodes, opto-electronic coupling elements, injection lasers. substrate crystals for epitaxial deposition with other compound semiconductors and mixed crystals such as, for instance, Ga(As,P) and as seed crystals for the drawing of, for example, gallium arsenide single-crystals by the protective-melt method has been shown. Although a specific example of the method has been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit of the invention which is intended to be limited solely by the appended claims.

What is claimed is:

1. A method for drawing a dislocation-free, singlecrystal gallium arsenide rod in a drawing apparatus comprising a closed quartz tube having at its bottom portion means to inductively heat a graphite crucible which is filled with gallium arsenide and placed in the bottom of the tube and means to support, pull and rotate a seed crystal comprising the steps of:

a. filling the quartz tube with an inert gas having a thermal conductivity which is less than or equal to 0.18 mW cm 'K at 300 K and less than or equal to 0.3 mW cm"K at l,500 K, at a pressure in the range of 0.2 to 0.8 Kg/cm b. depositing in said tube additional arsenic such as to maintain the partial pressure of the arsenic in the range of 0.8 to 1.5 [(g/cm during the drawing process;

c. inserting a seed crystal having a diameter in the range of 2 to 10 mm in the seed crystal holder;

d. inductively heating the graphite crucible to melt the gallium arsenide therein;

e. drawing a crystal of a length of approximately 2 cm from the melted gallium arsenide while maintaining the melt temperature at a level which will result in a crystal having a diameter as small as possible;

f. gradually reducing the melt temperature, while continuing the drawing, to cause an increase in crystal diameter until the desired crystal diameter is reached; and

g. continuing the drawing until the portion of the crystal having the desired diameter has reached the desired length.

2. The invention according to claim 1 wherein said inert gas comprises one of the group consisting of krypton, xenon and a mixture of krypton and xenon.

3. The invention according to claim 2 wherein one of the group consisting of argon, nitrogen and a mixture of argon and nitrogen are mixed with the inert gas.

4. The invention according to claim 1 wherein said tube is filled with an inert gas at a pressure of 0.5 kg/cm at room temperature.

5. The invention according to claim 1 wherein said additional arsenic is such as to maintain a partial pressure of 1.3 kg/cm 6. The invention according to claim 1 wherein the seed crystal has a diameter between 4 and 6 mm.

7. The invention according to claim 1 wherein said drawing apparatus further includes heating means surrounding the quartz tube and further including the step of heating said tube to maintain it at a temperature above that at which arsenic will precipitate on the walls ofthe tube. 

2. The invention according to claim 1 wherein said inert gas comprises one of the group consisting of krypton, xenon and a mixture of krypton and xenon.
 3. The invention according to claim 2 wherein one of the group consisting of argon, nitrogen and a mixture of argon and nitrogen are mixed with the inert gas.
 4. The invention according to claim 1 wherein said tube is filled with an inert gas at a pressure of 0.5 kg/cm2 at room temperature.
 5. The invention according to claim 1 wherein said additional arsenic is such as to maintain a partial pressure of 1.3 kg/cm2.
 6. The invention according to claim 1 wherein the seed crystal has a diameter between 4 and 6 mm.
 7. The invention according to claim 1 wherein said drawing apparatus further includes heating means surrounding the quartz tube and further including the step of heating said tube to maintain it at a temperature above that at which arsenic will precipitate on the walls of the tube. 